This invention relates to a test for evaluating corrosion and more particularly, to a corrosion evaluation testing method and apparatus for coated metallic material.
Generally, metallic material is coated on its surface with a film for preventing corrosion. The corrosion preventing performance of such coated film strongly depends on the resistance-polarization caused by the electric resistance of the coated film. Accordingly, various films of high resistance have been adapted to be coated on, for example, steel plate, etc. However, water still cannot be completely shut out even after application of a coating film of the above described type onto metallic material. Accordingly, corrosion progresses underneath the film. Also, when defects exist in the coated film, the corrosion in the defective portion of the film progresses more rapidly as the electric resistance of the film becomes higher. Thus, so-called "chipping corrosion" caused by film damage and cracks presents a serious problem.
In addition to the resistance-polarization described above, physicochemical effects can be caused under environmental conditions by a corrosive liquid; for example, preventing or promoting effects inherently caused by substances melted and discharged from the film, including pigments, must be taken into account.
At present, method of coated steel plate is tested for corrosion by a spontaneous or weathering exposure test, a dipping or immersing test, artificially accelerated exposure tests or the like. However, in the conventional methods, the specimens to be tested are first subjected to controlled damage and thereafter the corrosion results are simply observed visually; thus, the phenomenological mechanism relating the coated steel plate and to the testing method has not yet been sufficiently clarifed. Therefore, significant information for developing an improved film coating cannot be expected to be obtained with the conventional methods.
In addition, in response to an anodic corrosion of the coated steel plate, a cathodic reaction is in general caused in the neighborhood of the area wherein the above described anodic reaction is taking place. Thus, the corrosion phenomenon of hydrogen fragility corrosion or hydrogen cracking, etc. caused by the diffusion of atomic hydrogen, which is produced by the cathodic reaction, into the steel should not be overlooked from the view point of preventing the structure from being cracked by the corrosion phenomena. Furthermore, it is important to know the corrosion preventing capability of the film in corrosive liquids other than the electrolytes (for example, crude oil) and in soils or corrosive gases.
In order to quantitatively analyze the corrosion phenomenon relating to coated steel plate, first the phenomenological information of physicochemical quantities concerning the corrosion must be obtained. An electrochemical method by which the process of corrosion reaction can be easily traced is particularly suitable for this type of corrosion evaluation. However, since the film coated on the steel plate is high in resistance, a reliable measuring result necessary for estimating the corrosion reaction mechanism, cannot be obtained by the same electrochemical method used for measuring bare steel.
After much research the present inventors have developed a method based upon the potentiostatic electrolysis of the coated steel plate at the spontaneous electrode potential. The method includes a method of obtaining the polarization curve, a method of detecting the extremely small amount of current-potential variation or an infinitesimally small current-potential variation, and a method of detecting the film resistance specific to the coated film. Moreover, a method has been developed for detecting the electrolysis current of the discharge of the atomic hydrogen which is diffused into the non-film side from the film side; in the method the non-film coated side of the coated plate is potentiostatically electrolyzed at the spontaneous electrode potential. Thus, a basic object of the present invention is to establish a corrosion evaluation testing method for coated metallic material and associated apparatus. This comprises a method for detecting electrochemical information significant to the corrosion evaluation by subjecting coated metallic material to a series of electrochemical testing methods developed according to the present invention; there is also an apparatus for automatically performing the detecting method according to the type of specimen and evaluation needed.
Furthermore, up to the present, to evaluate corrosion at defects in a film coating conventionally a method of first providing cross-cuts on the film face of a coated specimen and then, visually observing the blistering and rust width near the cuts has been employed. However, with the conventional method, it is difficult to judge or confirm, from the outside, the corrosion forms, i.e., whether the corrosion tends to spread out under the film along the film defect portions ("crevice") or in depth below the original defect portions ("pitting"). Moreover, it is not practical to apply such corrosion prior art evaluating tests when the coated metallic objects are buried in water or in the soils. On the other hand, various electrochemical methods of measuring the electrode potential, etc. of the coated metallic object have also been conventionally performed. However, these prior methods are still insufficient for use in the corrosion evaluating test, because these electrochemical methods inherently involve complicated analyses.
Regarding the kinds of defects usually introduced in the conventional methods, the present inventors have found, after the various experiments on the polarization behavior of the coated metallic face by the use of the newly developed present method, that the peak portion will appear in the polarization curve when a coated metallic material whose film coat has defects is polarized in a cathodic mode. Also, the inventors' analysis, shows that a certain relationship can exist between the existence of a peak portion and the form of corrosion (corrosion form) in the film defect portion. Thus, the another object of the present invention is to provide an electrochemical method and an apparatus for detecting the corrosion form or structure in the film defect portion of the coated metallic plate material, so that this information can be used in the corrosion evaluation.
As described earlier, if the corrosion preventing performance of the film is properly quantitatively treated, it is not necessary to directly electrochemically measure the corrosion preventing performance of the film in the corrosive gases (such as steams), soils or non-electrolytes (such as crude oil).